The present invention relates to a fluid-fuel burner, especially for furnaces for reheating iron and steel products. The aim of this invention is to design a burner with a spread-out flame, by virtue of which the distribution of the heat flux generated by the flame is improved so as to reduce the temperature heterogeneity induced in the products to be reheated.
It is known that heat-treatment furnaces, especially reheat and soaking furnaces, are intended to raise products, especially slabs, blooms and the like, to the temperatures required, for example, for rolling or for the purpose of obtaining a given metallurgical structure.
It is also known that the quality of the treatment of a product, for example a rolling operation or a heat treatment, requires a precise and homogeneous temperature within the product, this temperature depending on the type of treatment desired or on the chemical composition of the product to be treated.
For example, in furnaces for reheating metallurgical products, the mean temperature level is obtained by passing the products through so-called heating zones which are characterized by a significant heat influx in a relatively short time, thereby generating considerable thermal heterogeneity in the reheated products. In order to obtain the temperature homogeneity required for their subsequent treatment, the products leaving the heating zones pass through an equalizing zone in which the heat influx is very small, thereby making it possible to equalize the temperatures within the products.
FIG. 1 of the drawings shows schematically, in side elevation and in vertical cross section, an illustrative example of a furnace of known type for reheating iron and steel products. This furnace is of the type with top and bottom heating.
As may be seen in FIG. 1, the products to be reheated, denoted by the reference 1, are supported and transported inside the furnace by a system of fixed beams 2 and walking beams 3, the walking beams 3 being moved by virtue of the combined actions of a translation frame 4 and of a lifting frame 5, giving the products 1 a back-step movement so as to convey them from the entry of the furnace to the exit. This is a system well known to those skilled in the art, which does not form part of the present invention and which is therefore unnecessary to describe in detail.
The furnace consists of a thermally insulated chamber 6, comprising respectively heating zones and equalizing zones, on which chamber top heating burners 7 and bottom heating burners 7xe2x80x2 are placed, as well as equalizing burners 8 fitted into the roof of the furnace, as may be clearly seen in FIG. 1. In this illustrative example, the burners of the heating zones 7 and 7xe2x80x2, which are fitted into the side walls of the furnace, are burners producing flames with axial development. The products to be treated are placed in a horizontal plane parallel to the axes of the burners. The latter may be fitted either in a plane lying above the plane of the bed of products (top burners 7) or in a plane lying below the plane of the bed of products (bottom burners 71). The height of the furnace chamber 6 is defined by the distance separating the plane of the products 1 from the sole of the furnace and by the distance separating the plane of the products from the roof of the furnace. This height depends on the characteristics and the dimensions of the flames from the burners 7 and 7xe2x80x2 fitted into the side walls of the furnace.
FIG. 2 of the drawings shows schematically the distribution of the heat flux generated by the burners 7, 7xe2x80x2 producing flames with axial development. In this Figure, the reference B denotes a burner, P the plane of the bed of products and I the image of the transmitted heat flux. It may be seen that the heat flux exhibits heterogeneities in the vertical planes perpendicular and parallel to the flame axis. These heterogeneities are caused by the progressive development of the combustion at the root of the flame or by the presence of hot zones in this flame, these being characteristic of this type of known burner.
In this illustrative example, the furnace also includes burners 8 fitted into the roof of the furnace in the equalizing zones of the latter. These are burners producing low-axial-momentum, high-swirl flames. The heat flux transmitted to the products to be reheated is constant, over a certain diameter, in a plane perpendicular to the axis of the burner 8 and parallel to the bed of products 1. These burners generate limited thermal heterogeneities in these products, however the area for homogeneous radiative exchange created by each burner is limited to a small unit area, thereby requiring a large number of burners such as 8 to be installed in order to ensure homogeneous reheating of the entire surface of the products. FIG. 3 shows schematically the burners 8 fitted into the furnace roof. In this figure, the reference Z denotes the main heating zone of each burner 8 on the furnace roof, Pv denotes the plane of the furnace roof and Ip denotes the image of the transmitted heat flux on the plane of the bed of products.
Finally, the bottom burners 7xe2x80x2, which are fitted below the plane of the products 1, cannot be replaced with burners producing low-axial-momentum, high-swirl flames since it is impossible for them to be fitted into the sole of the furnace because of the presence of the equipment for supporting the products and because of the droppings of part of the oxides which form on the surface of these products during reheating. The bottom burners such as 7xe2x80x2 of the equalizing zones can therefore only be burners producing flames with axial development, despite the heat-flux distribution shortcomings inherent in this type of burner.
As clearly follows from the above, achieving good temperature homogeneity of the reheated products in a furnace for reheating metallurgical products or in a heat-treatment furnace is limited by the current technology of burners of the type producing axisymmetric flames with axial development, or good temperature homogeneity is possible only partially by the complex and expensive installation of a large number of burners of the type producing low-axial-momentum, high-swirl flames on the furnace roof.
Starting from this state of the art, the present invention aims to provide a burner limiting the heat-flux distribution heterogeneities in vertical planes parallel and perpendicular to the flame axis by spreading out the area for exchange between the flame and the plane of the bed of products to be treated.
In order for the result made possible by the present invention to be clearly understood, reference is made to FIG. 4 which shows the distribution of the heat flux of the flame of a burner produced according to the arrangements of the present invention. As in FIG. 2, the reference B denotes the burner, the reference P denotes the plane of the bed of products and the reference I denotes the image of the transmitted heat flux. It is clearly apparent from FIG. 4 that the burner according to the invention develops a flame spread out parallel to the bed of products and the flux of which is preferably in the plane of the major axis of symmetry of the tile, parallel to the plane of the products. Comparing FIG. 4 with FIG. 2, commented on above, it clearly shows the technical progress provided by the invention vis-a-vis the burners according to the prior art.
The result thus illustrated is achieved by a burner according to the present invention which is essentially characterized in that it is provided with a combustion tunnel having a broad shape, provided with oxidizer and fuel injection orifices, these being approximately parallel to the major axis of symmetry of the tile, the internal shape of the latter as well as the orientation of the fuel and oxidizer injection orifices being chosen so as to create a difference in the distribution of the combustion products and of the recycled flue gases, producing a spread-out flame which ensures homogeneous distribution of the heat flux.
According to the present invention, the said combustion tile has a rectangular or oval shape, or any combination of these two shapes.
According to another characteristic of the invention, the axes of the oxidizer and/or fuel injection orifices lie in planes approximately parallel to the plane of the products to be treated.
According to one non-limiting embodiment of the invention, the burner includes:
on the one hand, an oxidizer feed provided with injection channels emerging in the combustion tile via the injection orifices, the latter being distributed around the burner axis and having axes which lie in planes approximately parallel to the plane of the products to be treated, and
on the other hand, a fuel feed located centrally and provided with injection channels which are distributed around the burner axis, their axes lying in planes approximately parallel to the plane of the products.
According to the invention, the injection channels emerge in the tile via the fuel injection orifices or, in the case in which the invention is applied to burners having separate fuel injection pipes, the fuel injection channels and orifices are located in the injection pipe.
These characteristics result in the gases having a low or zero swirl, thereby ensuring that the heat flux is distributed over a large area (or conversely), the flame at the exit of the combustion tile developing preferably in the plane of the major axis of symmetry of the tile, approximately parallel to the plane of the products.
According to another embodiment, the burner includes means for modulating the area over which the flame of the burner is distributed, it being possible for these means to be produced by delivering the oxidizer and/or the fuel in at least two separate groups.
The subject of the invention is also a furnace provided with burners having the characteristics defined above, especially a furnace for reheating iron and steel products. This furnace may include a radiating wall or exchange wall, located approximately parallel to the plane of the products to be treated, facing the flame spread plane of the burners. These burners are located so that the flame spread plane is approximately parallel to one of the walls of the furnace. According to the invention, the burners may be fitted into the side walls of the furnace, into at least one of its front walls, above and/or below the plane of the products to be treated.
Other features and advantages of the present invention will emerge from the description given below with reference to the appended drawings which illustrate embodiments of the invention, these being devoid of any limiting character.